Field of the Invention:
The present invention relates to improvements in phased arrays of dipole, or dipole-like radiating elements. It is of particular utility in two-dimensional, or so-called M .times. N arrays.
Description of the Prior Art:
One of the important performance criteria for electronically scanned dipole arrays is the variation of input impedance with scan angle. The larger the variation, the greater the reflected power with its attendant loss. In prior art apparatus, these variations have been the cause of significant losses. For example, over a 60-degree scan angle, the impedance variation is about six to one for E-plane scan and nearly four to one for H-plane scan. Since these occur in different directions, an optimization of source impedance results in a peak voltage standing wave ratio (VSWR) of nearly 5. This causes a loss due to reflected energy of about 2.5 db at the worst scan angle.
There are several known prior art techniques which reduce the impedance variation under E-plane scan, making it less than the H-plane variation, and/or cause it to occur in the same direction as the H-plane variation. One of these modifications is the bending of the dipole so that the outer tips are closer to the ground plane than the center. Another involves putting baffles between adjacent dipoles. The improvement provided by these techniques is considerable, for example resulting in a reduction of the VSWR to about 2.5, yielding a maximum loss of about 0.88 db. However, as far as known there is nothing in the prior art which gives any significant improvement in the H-plane impedance variation.
While there has been no known technique of reducing variation of impedance under H-plane scan, the techniques of controlling directivity pattern characteristics by interjecting metallic discs, rods or plates in the field adjacent the dipole radiating element is disclosed in prior art patents. One patent which is notable for its physical resemblance to the present invention is U.S. Pat. 3,742,513 FIG. 4 thereof discloses reflector discs above the radiating element of an array. However the principle of that patent is the achievement of directivity through the use of the the discs as backfire reflectors. Also, a rim, of variable length, comes out of the ground plane, forming a cavity for enhancing the beam directivity. The size of the reflector disc is not specified, but in accordance with the well known principles of design of backfire antennas, it is presumed to have a diameter of .lambda./2.
Also of interest are prior art patents which disclose discs, or plates in connection with a single radiating element (and not in an array organization). These include U.S. Patents 3,774,223 and 2,671,855 disclosing disk reflectors of .lambda./2 and 0.8 .lambda. diameter, respectively, acting as backfire reflectors for cavity or parabolic main reflecting surfaces; Patent 2,429,640, which discloses a rectangular plate having transverse dimensions which are multiples of .lambda./2 acting as a backfire reflector into a parabolic directive reflector; 3,483,043 and 3,508,278 which disclose reflector discs of .lambda./2 diameter, (or in the case of 3,508,278 the alternative having 1/10 of the main reflector area in a backfiring reflector); and 2,759,182 which discloses a paraboloidal curved rectangular plate having a major dimension 0.96 .lambda. and a minor dimension of 0.63 .lambda. in the backfiring reflector configuration. In all instances the dimensions are either specified to be in excess of .lambda./2 or may be presumed to be equal to or greater than .lambda./2 from well known design principles for backfiring reflectors. The function of the disk, rods or plates in all these cases is to achieve directivity.
Also of interest are various patents in which discs or plates are used as non-active directors in forming a beam as a connection with a dipole radiating element. Patent 3,821,745 discloses an assembly of a bent plate having a linear dimension between 1/4.lambda. and 1/2.lambda., and a rectangular planar plate having a dimension 1/3.lambda. which act as parasitic directors of the beam pattern; 2,556,046 discloses a disc having a diameter just under .lambda./2 which serves as a phase modifying driver; and 3,524,191 which discloses a yagi-type array composed of a multiplicity of elements for phase shift control to form an end-fire directive beam. Again, the sole function of the non-active elements is to form a beam. Except with regard to the yagi-type array of 3,524,191, the elements have major dimensions of .lambda./2 or more.
Prior to the present invention there has been no known use in dipole arrays of non-active disc or plate elements located beyond a dipole radiating element from the ground plane or reflector which serve to reduce variation of impedance of scan angle.